Anna University Long Questions and Answers

Unit - II

Chapter 2

Nanochemistry

Anna University Long Questions & Answers

 Part - B

1. Explain about size dependent properties of nano-materials. (A.U. (CEG) Dec 2012, June 2014)

Nearly all the properties as shown in the following figure, like hardness, strength, ductility, melting point and density, change for nanomaterials. These behaviors vary so significantly by a mere reduction in grain size. Nanomaterials are composed of grains and grain boundaries. Nanometre sized grains contains only a few thousands of atoms with in each grain.

A large number of atoms reside at the grain boundaries. As the grain size decreases, there is a significant increase in the volume fraction of grain boundaries or interfaces.

The properties of the materials are bound to be governed to a large extent by defect configurations. Hence the mechanical and chemical properties of nanomaterials are significantly altered due to defect dynamics. The elastic property of nanomaterials are different from that of bulk alloys due to the presence of increased fraction of defects.

Example

1. Nanocrystalline ceramics are tougher and stronger than those with coarse grains.

 

2. Write an informative note on the properties of nano-particles. (A.U. (CEG) June 2013)

(Or)

Discuss any four salient properties of nano-materials. (A.U. June 2014, Dec 2015)

1. Electrical Properties

(i) Electrical conductivity decreases with a reduced dimension due to increased surface scattering. However, it can be increased, due to better ordering in micro-structure.

Example : Polymeric fibres.

(ii) Nanocrystalline materials are used as very good separator plates in batteries, because they can hold more energy than the bulk materials.

Example

Nickel-metal hydride batteries made of nanocrystalline nickel and metal hydride, require far less frequent recharging and last much longer.

2. Optical Properties

Reduction of material dimensions has pronounced effects on the optical properties. Optical properties of nano-materials are different from bulk forms.

The change in optical properties is caused by two factors

(i) The quantum confinement of electrons within the nano-particles increases the energy level spacing.

Example

The optical absorption peak of a semiconductor nano-particles shifts to a short wavelength, due to an increased band gap.

(ii) Surface plasma resonance, which is due to smaller size of nano-particles than the wavelength of incident radiation.

Example

The colour of metallic nano-particles may change with their sizes due to surface plasma resonance.

3. Mechanical properties

The nano-materials have less defects compared to bulk materials, which increases the mechanical strength.

(i) Mechanical properties of polymeric materials can be increased by the addition of nano-fillers.

(ii) As nano-materials are stronger, harder and more wear resistant and corrosion resistant, they are used in spark plugs.

Example:

Nano-crystalline carbides are much stronger, harder and wear resistant and are used in micro drills.

4. Magnetic properties

Magnetic properties of nano materials are different from that of bulk materials. Ferro-magnetic behaviour of bulk materials disappear, when the particle size is reduced and transfers to super-paramagnetics. This is due to the huge surface area.

 

3. Discuss the laser ablation and CVD techniques for the synthesis of nanoparticles. (A.U (CEG) June 2013, A.U. Jan 2015)

(i) Laser ablation

In laser ablation technique, high-power laser pulse is used to evaporate the material from the target. The stoichiometry of the material is protected in the interaction.

The total mass ablated from the target per laser pulse is referred as the ablation rate.

This method involves vapourisation of target material containing small amount of catalyst (nickel or cobalt) by passing an intense pulsed laser beam at a higher temperature to about 120°C in a quartz tube reactor. Simultaneously, an inert gas such as argon, helium is allowed to pass into the reactor to sweep the evaporated particles from the furnace to the colder collector.

Fig. Laser ablation chamber

(ii) Chemical Vapour Deposition (CVD)

This process involves conversion of gaseous molecules into solid nanomaterials in the form of tubes, wires or thin films. First the solid materials are converted into gaseous molecules and then deposited as nanomaterials.

Fig. Chemical vapour deposition

Example: CNT preparation.

The CVD reactor consists of a higher temperature vacuum furnace maintained at inert atmosphere. The solid substrate containing catalyst like nickel, cobalt, iron supported on a substrate material like, silica, quarts is kept inside the furnace. The hydrocarbons such as ethylene, acetylene and nitrogen cylinders are connected to the furnace. Carbon atoms, produced by the decomposition at 1000°C, condense on the cooler surface of the catalyst.

As this process is continuous, CNT is produced continuously.

 

4. What are nano-particles? Write any four methods of preparation of nano-particles? (A.U. (CEG) Dec 2011)

(Or)

Discuss various types of synthesis involved in the preparation of nano-materials. (A.U. Jan 2014, June 2014)

Definition

Nanoparticles are the particles, the size of which ranges from 1-50 nm.

1. Solvothermal Synthesis

Solvothermal synthesis involves the use of solvent under high temperature (between 100°C to 1000°C) and moderate to high pressure (1 atm to 10,000 atm) that facilitate the interaction of precursors during synthesis.

Method

A solvent like ethanol, methanol, 2-propanol is mixed with certain metal precursors and the solution mixture is placed in an autoclave kept at relatively high temperature and pressure in an oven to carry out the crystal growth. The pressure generated in the vessel, due to the solvent vapour, elevates the boiling point of the solvent.

Example: Solvothermal synthesis of zinc oxide

Solvothermal synthesis of zinc oxide

Zinc acetate dihydrate is dissolved in 2-propanol at 50°C. Subsequently, the solution is cooled to 0°C and NaOH is added to precipitate ZnO. The solution is then heated to 65°C to allow ZnO growth for some period of time. Then a capping agent (1-dodecanethiol) is injected into the suspension to arrest the growth. The rod shaped ZnO nano-crystal is obtained.

Fig. Solvothermal synthesis

2. Laser ablation

In laser ablation technique, high-power laser pulse is used to evaporate the material from the target. The stoichiometry of the material is protected in the interaction. .. The total mass ablated from the target per laser pulse is referred to as the ablation rate.

This method involves vapourisation of target material containing small amount of catalyst (nickel or cobalt) by passing an intense pulsed laser beam at a higher temperature to about 120°C in a quartz tube reactor. Simultaneously, an inert gas such as argon, helium is allowed to pass into the reactor to sweep the evaporated particles from the furnace to the colder collector.

Fig. Laser ablation chamber

Uses

1. Nanotubes having a diameter of 10 to 20 nm and 100 um can be produced by this method.

2. Ceramic particles and coating can be produced.

3. Other materials like silicon, carbon can also be converted into nanoparticles by this method.

3. Chemical Vapour Deposition (CVD)

This process involves conversion of gaseous molecules into solid nanomaterials in the form of tubes, wires or thin films. First the solid materials are converted into gaseous molecules and then deposited as nanomaterials.

Example: CNT preparation.

The CVD reactor consists of a higher temperature vacuum furnace maintained at inert atmosphere. The solid substrate containing catalyst like nickel, cobalt, iron supported on a substrate material like, silica, quarts is kept inside the furnace. The hydrocarbons such as ethylene, acetylene and nitrogen cylinders are connected to the furnace. Carbon atoms, produced by the decomposition at 1000°C, condense on the cooler surface of the catalyst.

As this process is continuous, CNT is produced continuously.

Fig. Chemical vapour deposition

4. Electro-deposition (or) Electrochemical deposition

Electro-deposition is an electrochemical method in which ions from the solution are deposited at the surface of cathode.

Process of electro-deposition

The cell consists of a reference electrode, specially designed cathode and anode. All these electrodes are connected with the battery through an voltmeter and dipped in an electrolytic solution of a soluble metal as shown in figure. When the current is passed through the electrodes of template, the metal ions from the solution enter into the pores and gets reduced at the cathode, resulting in the growth of nanowire inside the pores of the template.

Fig. Electrodeposition method

 

5. Explain the preparation of nano-materials by (i) Sol-Gel process (ii) Electrospinning

1. Sol-Gel process

The sol-gel process is a wet chemical technique also known as chemical solution deposition. It is the method for producing solid materials from small molecules. This method is used for the fabrication of metal oxides. It involves conversion of monomers into a colloidal solution (sol), that acts as the precursor. This colloidal solution gradually evolves towards the formation of a gel-like system.

It involves the following steps.

1. Hydrolysis and polycondensation 2. Gelation 3. Aging 4. Drying 5. Densification 6. Crystallization

The volume fraction of particles (particle density) may be slow that a significant amount of fluid need to be removed for the gel-like properties to be recognized. It is done by two ways.

Fig. Various steps of Sol-Gel process

 (i) Sedimentation

The solution is allowed to keep for some time for sedimentation to occur and then pour off the remaining liquid.

(ii) Centrifugation

Centrifugation can also be used to accelerate the process of phase separation.

Drying and densification

Removal of the remaining liquid (solvent) is done by drying process, which accompanied by shrinkage and densification.

Firing (or) crystallization

A thermal treatment (firing) is necessary to enhance mechanical properties and structural stability via sintering, densification

2. Electrospinning

Electrospinning is a method of producing ultrafine (in nanometers) fibres by charging and ejecting a polymer solution through a spinneret under a high-voltage electric field and to solidify (or) coagulate it to form a filament.

Components

1. A high voltage power supply.

2. A polymer reservoir that can maintain a constant flow rate of solution.

3. A conductive needle, as polymer source, connected to the high voltage power supply.

4. A conductive collector (plate, drum, etc.)

Process

A polymer is dissolved in a suitable solvent and is filled in the capillary reservoir. When sufficiently high voltage is applied to create an electric field between the needle tip and the collector, a charge accumulates at the liquid surface. When the electrostatic repulsion is higher than the surface tension the liquid meniscus is deformed into conically shaped structure known as a Taylor cone.

Once the Taylor cone is formed, the charged liquid jet is ejected towards the collector. Depending upon the viscosity of the solution, solid fibre will be formed as the solvent evaporates.

 

6. Define the terms: nanorods, nanotubes, nanowires and nanoclusters. (A.U (CEG) June 2013)

(a) Nano-wires

Nanowire is two dimensional cylindrical solid material having an aspect ratio ie., length to width ratio greater than 20. Diameter of the nanowire ranges from 10 - 100 nm.

(b) Nano-rods

Nanorod is two dimensional cylindrical solid material having an aspect ratio i.e., length to width ratio less than

(c) Nano Cluster

Nanoclusters are fine aggregates of atoms or molecules. The size of which ranges from 0.1 to 10 nm. Of all the nano materials, nanoclusters are the smallest sized nano materials because of their close packing arrangement of atoms.

 (d) Nano-tubes

Nanotubes are tube like structures with diameter of 1 - 100 nm and a length of few nm to microns. Nanotubes consist of tiny cylinders of carbon and other materials like boron nitride. Nanotubes may be organic (or) inorganic.

 

7. What are nano materials? Discuss the types of carbon nano tubes and their applications. (A.U June 2012)

Nanomaterials are the materials having components with size less than 100 nm at least in one dimension.

Types of carbon nanotubes

(i) Single - walled nanotubes (SWNTS)

SWNTs consist of one tube of graphite. It is one-atom thick having a diameter of 2 nm and a length of 100 um. SWNTs are very important, because they exhibit important electrical properties. It is an excellent conductor.

Three kinds of nanotubes are resulted, based on the orientation of the hexagon lattice.

(a) Arm-chair structures: The lines of hexagons are parallel to the axis of the nanotube.

(b) Zig-zag structures: The lines of carbon bonds are down the centre.

(c) Chiral nanotubes: It exhibits twist or spiral around the nanotubes.

It has been confirmed that arm-chair carbon nanotubes are metallic while zig-zag and chiral nanotubes are semiconducting

(ii) Multi - walled nanotubes (MWNTs)

MWNTs (nested nanotubes) consist of multiple layers of graphite rolled in on themselves to form a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane.

Applications of Carbon Nanotubes

(a) It is used in battery technology and in industries as catalyst.

(b) It is also used as light weight shielding materials for protecting electronic equipments.

(c) CNTs are used effectively inside the body for drug delivery.

(d) It is used in composites, ICs..

 

8. How are carbon nano tubes synthesized? What are its applications? (A.U (CEG) Dec 2011, AU. Jan 2014, Jan 2015)

Carbon nanotubes can be synthesized by any one of the following methods.

(i) Pyrolysis of hydrocarbons.

(ii) Laser evaporation.

(i) Pyrolysis

Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 700°C in the presence of Fe-silica or Fe-graphite catalyst under inert conditions.

(ii) Laser evaporation

It involves vapourization of graphite target, containing small amount of cobalt and nickel, by exposing it to an intense pulsed laser beam at higher temperature (1200°C) in a quartz tube reactor. An inert gas such as argon is simultaneously allowed to pass into the reactor to sweep the evaporated carbon atoms from the furnace to the colder copper collector, on which they condense as carbon nanotubes.

Applications of CNTs

Fig. Laser evaporation technique

 

9. Explain (a) Nanocluster (b) Nanowire with examples (A.U Jan 2014)

(a) Nanocluster

Nanoclusters are fine aggregates of atoms or molecules. The size of which ranges from 0.1 to 10 nm. Of all the nano materials, nanoclusters are the smallest sized nano materials because of their close packing arrangement of atoms.

Examples: CdS, ZnO, etc.,

All the atoms, in nanocluster, are bound by forces like metallic, covalent, ionic, hydrogen bond' or Vander Waals forces of attraction. Clusters of certain critical size are more stable than others. Nanoclusters consisting of up to a couple of hundred atoms, but larger aggregates, containing 10° or more atoms, are called nanoparticles.

Magic number

Magic number is the number of atoms present in the clusters of criticle sizes with higher stability.

Production of Nanoclusters

Nanoclusters can be produced from atomic (or) molecular constituents (or) from the bulk materials either by bottom up process (or) top down process.

Atomic clusters (or) molecular clusters are formed by the nucleation of atoms (or) molecules respectively.

Properties of nanoclusters

(i) The reactivity of nanoclusters are decreased due to their decrease in size.

(ii) The melting point of nanoclusters are lower than the bulk materials due to high surface to volume ratio.

Applications of nanocluster

(i) Nanoclusters are used as catalysts in many reactions.

(ii) It is used in nano based chemical sensors.

(b) Nanowire with examples

Nanowire is one dimensional cylindrical solid material having an aspect ratio ie., length to width ratio greater than 20. Diameter of the nanowire ranges from 10 - 100 nm.

Synthesis of nanowires

Template-assisted synthesis

Template assisted synthesis of nanowires is a simple way to fabricate nanostructures. These templates contain very small cylindrical pores or voids within the host material and the empty spaces are filled with the chosen material to form nanowires.

Properties of nanowires

(i) Nanowires are two-dimensional material.

(ii) Conductivity of a nanowire is less than that of the corresponding bulk materials.

Uses of nanowires

(i) Nanowires are used for enhancing mechanical properties of composites.

(ii) It is also used to prepare active electronic components such as p-n junction and logic gates.

 

10. Discuss the solvo thermal and laser ablation methods of synthesis of nano-materials. (A.U. May 2015) Refer Q.No. 4 Page No B5.

Definition

Nanoparticles are the particles, the size of which ranges from 1-50 nm.

1. Solvothermal Synthesis

Solvothermal synthesis involves the use of solvent under high temperature (between 100°C to 1000°C) and moderate to high pressure (1 atm to 10,000 atm) that facilitate the interaction of precursors during synthesis.

Method

A solvent like ethanol, methanol, 2-propanol is mixed with certain metal precursors and the solution mixture is placed in an autoclave kept at relatively high temperature and pressure in an oven to carry out the crystal growth. The pressure generated in the vessel, due to the solvent vapour, elevates the boiling point of the solvent.

Example: Solvothermal synthesis of zinc oxide

Solvothermal synthesis of zinc oxide

Zinc acetate dihydrate is dissolved in 2-propanol at 50°C. Subsequently, the solution is cooled to 0°C and NaOH is added to precipitate ZnO. The solution is then heated to 65°C to allow ZnO growth for some period of time. Then a capping agent (1-dodecanethiol) is injected into the suspension to arrest the growth. The rod shaped ZnO nano-crystal is obtained.

Fig. Solvothermal synthesis

2. Laser ablation

In laser ablation technique, high-power laser pulse is used to evaporate the material from the target. The stoichiometry of the material is protected in the interaction. .. The total mass ablated from the target per laser pulse is referred to as the ablation rate.

This method involves vapourisation of target material containing small amount of catalyst (nickel or cobalt) by passing an intense pulsed laser beam at a higher temperature to about 120°C in a quartz tube reactor. Simultaneously, an inert gas such as argon, helium is allowed to pass into the reactor to sweep the evaporated particles from the furnace to the colder collector.

Fig. Laser ablation chamber

Uses

1. Nanotubes having a diameter of 10 to 20 nm and 100 um can be produced by this method.

2. Ceramic particles and coating can be produced.

3. Other materials like silicon, carbon can also be converted into nanoparticles by this method.

3. Chemical Vapour Deposition (CVD)

This process involves conversion of gaseous molecules into solid nanomaterials in the form of tubes, wires or thin films. First the solid materials are converted into gaseous molecules and then deposited as nanomaterials.

Example: CNT preparation.

The CVD reactor consists of a higher temperature vacuum furnace maintained at inert atmosphere. The solid substrate containing catalyst like nickel, cobalt, iron supported on a substrate material like, silica, quarts is kept inside the furnace. The hydrocarbons such as ethylene, acetylene and nitrogen cylinders are connected to the furnace. Carbon atoms, produced by the decomposition at 1000°C, condense on the cooler surface of the catalyst.

As this process is continuous, CNT is produced continuously.

Fig. Chemical vapour deposition

4. Electro-deposition (or) Electrochemical deposition

Electro-deposition is an electrochemical method in which ions from the solution are deposited at the surface of cathode.

Process of electro-deposition

The cell consists of a reference electrode, specially designed cathode and anode. All these electrodes are connected with the battery through an voltmeter and dipped in an electrolytic solution of a soluble metal as shown in figure. When the current is passed through the electrodes of template, the metal ions from the solution enter into the pores and gets reduced at the cathode, resulting in the growth of nanowire inside the pores of the template.

Fig. Electrodeposition method

 

11. Compare the properties of molecules, nanoparticles and bulk materials. (A.U May 2015)

(or)

Distinguish molecules, nanoparticles and bulk materials. (A.U Jan 2014)

 

12. (a) Write short notes on: (i) Carbon nanotubes (ii) Nanorods (iii) Nanowires (A.U Dec 2015)

(i) Carbon nanotubes

Nanomaterials are the materials having components with size less than 100 nm at least in one dimension.

Types of carbon nanotubes

(i) Single - walled nanotubes (SWNTS)

SWNTs consist of one tube of graphite. It is one-atom thick having a diameter of 2 nm and a length of 100 um. SWNTs are very important, because they exhibit important electrical properties. It is an excellent conductor.

Three kinds of nanotubes are resulted, based on the orientation of the hexagon lattice.

(a) Arm-chair structures: The lines of hexagons are parallel to the axis of the nanotube.

(b) Zig-zag structures: The lines of carbon bonds are down the centre.

(c) Chiral nanotubes: It exhibits twist or spiral around the nanotubes.

It has been confirmed that arm-chair carbon nanotubes are metallic while zig-zag and chiral nanotubes are semiconducting

(ii) Multi - walled nanotubes (MWNTs)

MWNTs (nested nanotubes) consist of multiple layers of graphite rolled in on themselves to form a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane.

Applications of Carbon Nanotubes

(a) It is used in battery technology and in industries as catalyst.

(b) It is also used as light weight shielding materials for protecting electronic equipments.

(c) CNTs are used effectively inside the body for drug delivery.

(d) It is used in composites, ICs..

(ii). Nanorods

Nanorod is two dimensional cylindrical solid material having an aspect ratio i.e., length to width ratio less than 20.

Synthesis of nanorods

Nano-rods are produced by direct chemical synthesis. A combination of ligands act as shape control agents and bond to different facets of the nano-rods with different strength.

This allows different nanorods to grow at different rates producing an elongated objects. Many of the above nanorods are not manufactured due to lack of commercial demand.

Properties of nanorods

1. Nanorods are two-dimensional materials.

2. It exhibits optical and electrical properties.

Applications of nanorods

1. Nanorods find application in display technologies.

2. It is also used in the manufacturing of micro mechanical switches.

3. They are used in energy harvesting and light emitting devices.

 4. Nanorods have used as cancer therapeutics.

(iii) Nanowires

Nanowire is two dimensional cylindrical solid material having an aspect ratio ie., length to width ratio greater than 20. Diameter of the nanowire ranges from 10 - 100 nm.

Synthesis of nanowires

1. Template-assisted synthesis

Template assisted synthesis of nanowires is a simple way to fabricate nanostructures. These templates contain very small cylindrical pores or voids within the host material and the empty spaces are filled with the chosen material to form nanowires.

2. VLS (Vapour - Liquid - Solid) method

It involves the absorption of the source material from the gas phase into a liquid phase of catalyst. Upon supersaturation of the liquid alloy, a nucleation event generates a solid precipitate of the source material. This seed serves as a preferred site for further deposition of material at the interface of the liquid droplet, promoting the elongation of the seed into a nanowire.

Properties of nanowires

1. Nanowires are two-dimensional material.

2. Conductivity of a nanowire is less than that of the corresponding bulk materials.

3. Silicon nanowires show strong photoluminescence characteristics.

Uses of nanowires

1. Nanowires are used for enhancing mechanical properties of composites.

2. It is also used to prepare active electronic components such as p-n junction and logic gates.

 

13. Explain briefly the applications of nano-materials. (A.U.T (TNV) Jan 2009)

(Or)

Explain any sir applications of nano-materials in various fields. (A.U. May 2014)

 

I. Medicine

1. Nano drugs

Nano materials are used as nano drugs for the cancer and TB therapy,

2. Laboratories on a chip

Nano technology is used in the production of laboratories on a chip.

3. Nano-medibots

Nano particles function as nano-medibots that release anti-cancer drug and treat cancer.

4. Gold-coated nanoshells

It converts light into heat, enabling the destruction of tumours.

 

II. In Agriculture

1. They also minimize the amount of harmful chemicals that pollute the environment.

2. Nanosensors are used in crop protection for the identification of diseases and residues of agrochemicals.

3. Nanodevises are used for the genetic engineering of plants.

 

III. In Energy

1. Power generation

Sun light, concentrated on nanoparticles, can produce steam with high energy efficiency, which can even be used in running power plants.

2. Generating hydrogen from sea water

The use of a nanostructured thin film of nickel selenide as a catalyst for the electrolysis of hydrogen from sea water.

3. Producing high efficiency light bulbs

Nano-engineered polymer matrix is used for the production of high efficiency light bulbs.

 

IV. Electronics

1. Quantum wires are found to have high electrical conductivity.

2. The integrated memory circuits have been found to be effective devices.

3. Nano wires are used to build transistors without p-n junctions.

 

V. In Catalysis

1. Water purification

Nanosilver catalyst is highly efficient in controlling microbes in water.

2. Bio-diesel production

Solid base nanocatalyst KF/CaO can be used for biodiesel production with yield more than 96%.

3. Fuel cell application

Carbon supported electro-catalysts play an important role in fuel cell.

4. Gold nanoparticles

It is an important catalyst in co-oxidation, epoxidation of propylene, hydrogenation of unsaturated hydrocarbons.